Aluminum alloys, such as aluminum-magnesium alloys are commonly subjected to various heat treatments in their processing. Abbe, in U.S. Pat. No. 2,379,467, recognized in 1945 that such heat treatment can cause surface blistering. Abbe solved this problem by cleaning aluminum alloy forgings with lye and nitric acid solution, and then wetting them with a water solution of sodium fluorborate, and then drying the forgings before heat treatment. Even earlier, Stroup, in U.S. Pat. No. 2,092,033, recognized that there was a greater susceptibility to blistering where the atmosphere surrounding the aluminum contained constituents such as water vapor, ammonia or sulfur compounds and where the aluminum was alloyed with magnesium in combination with copper, nickel, silicon and/or zinc. Stroup found that most blistering occurred at temperatures over about 427° C. (801° F.)—above the usual range employed for annealing cold worked aluminum alloys. Stroup's solution to eliminate blistering was to provide a protective atmosphere containing vaporous fluorine-bearing compounds, based on the volume of the heating furnace. The fluorine constituent was added to the furnace atmosphere either by heating solid fluorine-containing salts in the furnace or by introducing vaporous fluoride such as hydrogen fluoride, boron fluoride, or low molecular weight or low carbon number fluorinated aliphatic hydrocarbons in gaseous form. In the former case, the solid materials may be deposited on the floor of the furnace or in some container and allowed to vaporize or decompose under the influence of heat. Certain fluorides, calcium fluoride, sodium fluoride and potassium fluoride, were not found to be effective, however. In U.S. Pat. No. 2,380,202 (Stroup) the beneficial effect of a fluorine-containing atmosphere was improved, for alloys containing at least 70% magnesium, by incorporating calcium or beryllium into the alloy and using an atmosphere of HF, BF3, organic fluoride gases and dry carbon dioxide rather than air. Stroup, in U.S. Pat. No. 2,092,034 deliberately applied an artificial oxide coating to aluminum alloys before subjecting the alloy to thermal treatment, as a means to control high temperature blistering. These vapors could however cause corrosion problems in furnaces over a long period of time. In a subsequent article “Atmospheric Control In The Heat Treatment Of Aluminum Products”, P. T. Stroup, Controlled Atmospheres, October 1991, pp. 207-220, American Society for Metals, Cleveland, Ohio, Stroup further discussed and illustrated blistering and voids in cross-sectional views caused by heating aluminum sheet in air containing as little as 0.0012% total sulfur.
U.S. Pat. No. 2,885,313 (Milliken) teaches thermal treatment of aluminum base alloys containing magnesium to prevent subsequent atmospheric and high-temperature oxidation and corrosion and to facilitate degassing of finished and semi-finished articles of such alloys. Milliken postulated that the oxidation was caused by the attack of water upon the surface, oxidizing the aluminum and releasing hydrogen. There, finished or semi-finished aluminum base alloy articles were coated with an “organic ammonium fluoroborate—carrier compound” an amine or amide compound with boron trifluoride (that is a reaction product with the boron trifluoride) which would yield boron trifluoride, BF3, when heated for a time sufficient to cause substantial disappearance of the organic part of the organic ammonium fluoroborate compound, and provide a “protective film” on the alloy surface which film was substantially free of organic material. The Milliken organic ammonium fluoroborate-carrier compounds included aliphatic ammonium fluoroborates, di-n-butyl ammonium tetrafluoroborate; n-octadecyl-N,N,N, trimethyl tetrafluoroborate and di-n-amyl ammonium tetrafluoroborate. These organic ammonium fluoride compounds are applied generally as solutions in alcohol or water where the boron trifluoride content was from about 0.4 wt. % to preferably 2.6 wt. %. Then, as a separate heating step, degassing followed at from about 399° C. to 524° C. (750° F. to 975° F.) for from several hours to 20 days depending on the thickness of the article and rate of hydrogen diffusion in the particular alloy.
U.S. Pat. Nos. 2,885,315; 2,885,316; 2,969,590; 3,087,213; 6,120,618; 5,753,056; 5,985,059; and 6,355,121 B1 are additional, patents generally relating to this area. Iino et al., in U.S. Pat. No. 6,306,226 B1 also used a chemical etching process, while Ishii et al., in U.S. Pat. No. 4,671,825 formed a hydrophilic corrosion-resistant coating on aluminum alloy material. There an aqueous solution of dissolved water-soluble acrylic acid polymer, dispersed colloidal silica, hexavalent chromium compound, phosphoric acid, and fluorine compound were used.
The blistering and hydrogen degassing problems have never been completely solved. What is needed is a method that can apply a continuous film of protection and which will adhere to and cover corners, edges, and the bottom portion of aluminum articles during heat treatment. This is particularly important regarding aircraft components and the like.